For a while now, I've been forced to change "watch?v=" to "/embed/" to watch something in 480p on an i3 Gen 4, where the same video, when downloaded, uses ~3% of the CPU.
However, unfortunately, it doesn't always work anymore.
https://www.youtube.com/watch?v=xvFZjo5PgG0 https://www.youtube.com/embed/xvFZjo5PgG0
While they worsen the user experience, other sites optimize their players and don't seem to care about downloaders (pr0n sites, for example).